Catalytic oxidation of naphthalene



D. A. ROGERS ET AL A CATALYTIC OXIDATION OF NAPHTHALENE Filed Aug. 22, 1935 Feb. 23, 1937.

Passes Feb. 23, 1937 UNITED STATESv PATENT ori-ics 2,071,361 voA'rAIm'rrc OXIDATION or NAPHTHALENE Donald A. Rogers, Petersburg Va'.,and Frank Porter; Syracuse, N. Y., assignors, by mesne assignments, to The Solvay Process Company, Y Y New York, N. Y., a corporation of New York Application August, 22, 1935, serial No. 37,360

12 claims. (C1. 26o-123) This invention relates to the oxidation of naphthalene to produce partial oxidation products such as phthalic anhydride and maleic anhy temperature substantially in excess of that desired, resulting in increased total loxidation of the hydrocarbon and reduced yields of partial oxidation products. If it is attempted to increase dride. the yields `of partial oxidation products by main-` p Y 5 In the catalytic oxidation of naphthalene 4to taining the catalyst bed at atemperautre sub-'f5 partial oxidation products, as practiced prior to stantially below the desired maximum,` portions the present invention, the naphthalene is vaporof the catalyst bed may be overcooled so that subized and the vapors are mixed with air or an stantial' quantities of intermediate oxidation` equivalent oxidizing gas in a ratio of about one products are obtained inthe final product. Thus l part by Weight of naphthalene to 25 parts or 35 substantial quantities of naphthoquinone and' 10 parts by Weight of the oxidizing gas (representcoumarin are recovered from the reaction gases. ing a molar or volume ratio of from about 1:110 to In view of the close similarity of the physical 1:160). The mixture is passed at a temperature properties of `these compounds to those of of from 200 to 350 C. into a catalyst bed `conphthalic anhydride, it is exceedingly diihcult to l5 taining an oxidation catalyst such as vanadium effect their separation from the final phthalic an- 15 oxide (V205). Normally the oxidation catalyst hydride product.

' is disposed on a suitable carrier to increase its Itis an object of the present invention to overcontact area and avoid packing. come these disadvantages and not only to obtain While a hig-h temperature, say about 500 C., v high yields of the desired partial oxidation prodshould be maintained in the converterto insure uotS, but at the same time to obtain these prod- 20 the desired oxidation, the maximum temperature ucts free from naphthoquinone and coumarin imat which a commercially satisfactory yield of purities. lphthalic anhydride is obtained is about 600 C. As indicated above, in the catalytic oxidation At higher temperatures further oxidation is efof naphthalene as normally carried out, a Weight fected in marked degree converting the phthalic ratio of naphthalene to air-of around 1:30, which 25 anhydride to carbon dioxide and water and very corresponds to an amount of air substantially in materially reducing the output of desired prodexcess of the theoretical amount necessary for uct. Because of the exothermal character of the oxidation of naphthalene to phthalic anhydride, oxidation reaction, the temperature of the mixis employed. It might be supposed that by reducture passing through the catalyst bed is rapidly ing the proportion of air, overoxidation could be 304 raised and if some adequate cooling means were prevented. Such a process, however, is open to not provided, would greatly exceed the maximum at least two serious objections. In the rst place, temperature desired. Accordingly the converter suilicient reduction of the quantity of 'air emis generally designed as a heat exchanger proployed to Aexert a substantial effect on the total 3, viding extensive cooling area and the catalyst bed oxidation of naphthalene results in the formation 35 is cooled by indirect heat exchange with a coolof explosive mixtures. Employment of such mixing medium. tures in commercial operations would present The gas and vapor mixture resulting from the such a hazard that itsv use could not be counte-v above catalytic oxidation step is at a temperature nanced. In the second place, such reduction in 40 in the neighborhood of 450 or 550 C. and conthe proportion of air would result in the forma- 40 tains phthalic anhydride and by-products, f. i. tion of greater quantities of intermediate oxidamaleic anhydride, nitrogen, carbon dioxide, wation products such as naphthoquinone and couter, and perhaps coumarin and naphthoquinone marin and thus the purification process would be formed as intermediate oxidation products. Acfurther complicated.

cordingly the reaction gases are subjected to some o It has now been found that not only may the 45 sort of cooling process to remove condensable recooling diilculties encountered in the process action products. above outlined be avoided, but high yields of Maintenance of the requisite reaction temperaphthalic anhydride may be obtained substanture during oxidation by indirect cooling depends tially free from imuprities such as naphthoupon interchange of heat through the walls of the quinone and coumarin by employing the naph- 5o catalytic converter, and it is difiicult to obtain thalene in an oxidizing gas mixture in a molar uniform temperature conditions throughout the ratio of less than about 1:200,that is, the hycatalyst bed. If temperature conditions are not drocarbon vapor is mixed with more than about uniform, some portions of the bed may reach a 45 or `50 times its Weight of another gas or gases,

including the oxidizing constituent, before be- 55 ing introduced into the converter. The term oxidizing gas as used herein denominates a gas consisting of or containing oxidizing constituents, thus ogygen, air, or oxygen mixed with any inert diluent is covered by this term.

The required ratio of naphthalene to oxidizing gas may be obtained in several ways. The quantity of air mixed with the naphthalene vapors may merely be increased to about double that normally`employed. In this case it might be supposed that the increased quantity of oxidizing gas present would serve to further oxidize phthalic anhydride to carbon dioxide and water. However, it has been found that such is not the case and that although the ratio of oxygen to hydrocarbon is increased, nevertheless the increased quantity of air in the reaction mixture alters other factorsin the reaction process and inhibits oxidation to a degree beyond that necessary for formation of phthalic anhydride. As a second method of carrying out the process, a portion of the tail gases may be recirculated in sufficient quantity to increase the total amount of gas present to more than 45 or 50 times the amount of naphthalene in the mixture. Other inert gases may, of course, be employed for effecting this dilution with a corresponding result.

By reduction of the naphthalene content as mentioned above a suiicient volume of gas is provided in the converter so that the heat of the reaction is dissipated in raising the temperature of the gas mixture, and it is possible to carry out the catalysis substantially adiabatically, i. e. without substantial heat loss other than the normal radiation, conduction, and convection losses to the atmosphere. The temperature of the catalyst is in this Way maintained below the maximum reaction temperature desired even without any further cooling of the bed at all. Thus with a catalytic converter operating adiabatically a mixture of air and naphthalene vapor in a weight ratio of 65:1 may be introduced into the converter at a temperature of 325 C. without rise of temperature in the converter above about 550 C. Since the diluent is uniformly mixed with the reacting vapors a uniform cooling effect is exerted on the reaction and no local overheating can result.

In order to provide maximum uniformity of reaction conditions in the converter, it is preferred to provide the walls of the converter with heat insulation to reduce the cooling eiect of the surrounding atmosphere and to prevent local cooling of the catalyst near the converter walls. However, by suitable arrangement of the catalyst bed or beds so as to diversify the ow of reacting gases substantially uniform conditions may be obtained in the converter without the application of such insulation and it will be understood that the present invention comprehends such operation.- The extent of atmospheric cooling, of course, depends upon the size of the converter, the material of which it is composed, and other factors such as the temperature of the air in contact with it and the extent of circulation of the surrounding air. With oxidation catalysts of relatively low activity a large catalyst contact area must be provided and hence a large converter with correspondingly increased surface area exposed to the surrounding air would be employed. In spite of these possible variations in procedure, it is not necessary to subject the catalyst bed to controlled cooling to maintain the desired temperature since the temperature may be controlled merely by varying the proporyfor the apparatus.

tion of naphthalene in the reacting gases. Thus, for any specific type of catalyst and for any specic type of converter design uniform temperature conditions may readily be obtained.

In the oxidation of naphthalene by means of air alone a weight ratio of naphthalene to air between the limits 1:45 and 1:110 (representing a molar ratio between 1:200 and 1:500) is ordinarily advantageous. A weight ratio of from about 1:75 to 1:90, or molar ratio from about 1:330 to 11400, in particular gives optimum conditions when employing a converter in which no substantial heat loss takes place. The optimum naphthalene-air ratio may vary somewhat, depending upon the quality of the naphthalene employed and upon the nature of the gaseous diluent, for example, if tar is present with the naphthalene, the heat generated by the reaction is 'increased and accordingly the naphthalene:air ratio should be correspondingly decreased to cornpensate for the additional heat generated. The ratio of naphthalene to oxidizing gas will depend to some extent upon the specic heat of the gas employed. Hence the ratio of naphthalene to oxidizing gas may be varied depending upon the relative proportions of the various constituents in the gas mixture. While this consideration may have some slight eect upon the optimum ratio when a portion of the tail gas is recirculated to obtain the desired dilution, the tail gas would contain not only an increased nitrogen content but generally an increased carbon dioxide content. Since the specic heat of nitrogen is higher and that of carbon dioxide is lower than the specific heat of oxygen, the increased proportion of each of these constituents tends to compensate for the other. The optimum ratio also is affected by the temperature at which the mixture is introduced into the catalyst bed. Thus, while an inlet temperature of 325 C. has been mentioned, lower or higher inlet temperatures may be employed in which case a correspondingly increased or decreased ratio of naphthalene to oxidizing gas should be used. It is preferred to introduce the mixture into the catalyst at a temperature between 300" and 350 C.

In the preferred embodiment of the present invention the tail gases, i. e. the reaction gases from which the desired partial oxidation product has been removed are recirculated. It is preferred, too, to eect the catalytic oxidation at a moderately elevated pressure, say around 2 to 5 atmospheres absolute, although the invention is applicable to processes carried out at pressures higher or lower than these. By recirculation of tail gases and-by avoidance of unnecessary pressure drops in the circulation system only a fraction of the power required for compressing air is required for recompressing the gases to the preferred operating pressures and the elevated pressure makes possible a much greater throughput Not only do the tail gases recirculated perform admirably the function of absorbing the heat of reaction but by their recirculation the amount of gases exhausted to the atmosphere is reduced to only onethird or even less of that exhausted with the usual non-recirculatory systems. Such exhaust gases normally vcontain some small amounts of acid fumes and the quantity of these fumes will be more or less proportional to the amount of gas exhausted. Hence this fume nuisance is greatly reduced. The fumes exhausted with the gases are still further reduced in the preferred process by the fact that every step of the gas treatment is conducted under pressure and the condensation and elimination of vaporous constituents from the gases is thereby rendered more complete.

Partial oxidation of the hydrocarbon may be partly effected in a primary catalyst bed at normal conversion temperature giving reaction gases substantially free from hydrocarbon but containing intermediate oxidation products, and then completed in a second catalyst bed operating at a lower temperature, i. e. 350 to 450 C. as described in the application of Frank Porter, Serial No. 759,972, filed December 31, 1934. No cooling of either catalyst bed is required.

When the catalytic converter is operated at atmospheric pressure, it has been found that reduction of the oxygen content of the gas employed as the oxidizing medium below about 8% at the outlet end of the converter tends to cause reduction of the catalyst. At pressures of 2 atmospheres absolute or above, however, this reduction does not appear to take place until the oxygen is reduced below 5% at the outlet end of the converter, The pressure may be increased to a very high degree, for example as high as 300 pounds per square inch, if desired. However, in-

crease of pressure above 3 atmospheres absolute tends to narrow the temperature range for optimum yields of phthalic anhydride and therefore at higher pressure more accurate temperature control is required. Special equipment to stand the strain of combined high temperature and high pressure also is required. In view of these considerations the preferred pressure range of the present process is 2 to 5 atmospheres absolute and particularly about 3 atmospheres.

The following description of the preferred embodiment of our invention, apparatus for the operation of which is shown in the appended drawing, serves to further demonstrate the invention.

In the drawing numeral I designates a naphthalene vaporizer of any suitable type. The conventional vaporizer `is shown having a steam jacket 2, a gas inlet 3 near the bottom thereof and a naphthalene or hydrocarbon inlet 4. Vapor l conduit 5 leads from vaporizer I to heat exchanger 6, which may be of any suitable construction, providing indirect heat exchange between a pair of gas streams. A valved bypass 1 for gases is provided so that the vapor content of the mixture may be regulated by adding oxidizing gas in the event the gases leaving the vaporizer I contain more than the desired ratio of naphthalene vapor to oxidizing gas. From heat exchanger 6, vapor conduit 8 leads to a second heat exchanger 9 of the same or different type. A conduit Ill leads from heat exchanger 9 to a primary converter II.

Converter II may be of any suitable construction arranged for the passage of gases into intimate contact with solid catalyst and may be provided With one or a number of trays I2 for supporting the catalyst bed. The converter, of course, should be designed for operation at the elevated temperature and pressure at which it is preferred to carry out the process. Preferably the walls of the converter are suitably insulated to avoid heat loss and prevent local cooling of the catalyst near these` walls. The catalyst within the converter may consist of a bed of crushed alundum or silica brick having disposed thereon vanadium oxide in known manner.

From converter II, a conduit I3 leads to heat exchanger 9which serves the dual function of reducing the temperature of the reaction gases and heating the gas passing to the converter.

'I'he heat exchanger 9 is illustrated as composed of two sections, Ida. and Mib, although these sections may, of course, be entirely separate heat exchangers. Inlet I5 and outlet I6 are provided for flow of cooling fluid through section Mb of exchanger 9. From section Mb of heat exchanger 9 vapor conduit I1 leads to secondary converter I 8 of the same general type as converter II. This converter has an outlet conduit I9 leading to heat exchanger 6, which, as now will be apparent, serves to transfer heat from the hot reaction gas stream to the cooler gases passing to the converter II. From heat exchanger 6 vapor conduit leads through a distributor 2| into cooler 22.

Cooler 22 may be of any suitable construction providing intimate contact between hot reaction gases and aqueous phthalic acid slurry. 'I'he gas may be bubbled up through a body of liquid as in the case of the device illustrated or alternatively the liquid may be sprayed into the gas or streams of liquid and gas may be passed in intimate relation.

Removal of phthalic anhydride from the reaction' gases in the abovey system takes place in cooler 22 into which aqueous phthalic acid slurry is introduced. This serves not only to cool the gases but to scrub them so that practically all of the phthalic anhydride will be removed therefrom. The phthalic anhydride reacts with the water in the cooler to form phthalic acid. Any maleic anhydride contained in the gases is simultaneously scrubbed out by the water and forms an aqueous maleic acid solution.

As illustrated, the cooling chamber 22 oomprises a shell adapted to contain a liquid and having gas outlet conduit 23 at the top thereof and a sloped bottom 22a to permit withdrawal of slurry through outlet conduit without substantial separation of solid from liquid. Inlet conduit 20 is provided with the distributor 2I for uniformly distributing the gases beneath the surface of liquid in the cooler. In somev cases it may be desirable to provide insulation or steam jacketing around the exterior of the portions of conduit 20 within the cooler in order to avoid cooling of this conduit to such an extent that phthalic anhydride would condense on the interior surfaces of the conduit and cause clogging.

Gas conduit 23 having a valved bleed line 24 leads to recirculating pump 25, which in turn is connected toA gas conduit 3. On conduit 3 a branch conduit 26 from compressor 21 isy provided. This compressor serves to introduce oxidizing gas, for example air, into the system throughran inlet 28. A preheater 29 may be provided onthe inlet line 3 to raise the temperature of entering air suinciently to avoid cooling naphthalene below its melting point. It will be understood, however, that normally the heat of compression of the gases will be sufficient for this purpose.

From the bottom of cooler 22 an outlet pipe 30 and branch pipe 3| provided with a valve 32 lead to a slurry tank 33 which may to advantage be constructed similar to cooler 22 for reasons to be hereinafter more fully set forth. From tank y 33 a liquid withdrawal pipe 34 having a valve 35.

flash tank a vapor line 4I leads to a suitable evacuating means which, as shown in the drawing, may be a. jet evacuator and condenser 42 having a water inlet 43 at the top thereof and a. liquid outlet 44 at the bottom thereof. The outlet 44 may be sufficiently long to permit flow of liquid therethrough in opposition to air pressure or a pump may be provided for drawing it through this line. At the bottom of pipe 44 a liquid seal 45 with outlet 46 for suitable liquid disposal is illustrated.

Outlet conduit 30 from the direct contact cooler 22 is shown provided with a second withdrawal pipe 41 having a valve 41a thereon. Withdrawal pipe 41 leads to suitable separating means, such as a drum lter 48. The drum filter is provided with the customary doctor 49 for scraping solid therefrom and with a liquid outlet 50 leading to a separator 5I for eliminating entrained air. An air line 52 from the separator leads to a suitable exhauster (not shown) for supplying suction to the filter 48. At the bottom of the separator 5| a liquid withdrawal pipe 53, having a valved branch `54, is shown. Pipe 53 leads to the tank 33 for return of liquid from the filter to the cooling system. The branch 54 is a bleed off for maleic acid solution. Doctor 49 of the drum lter is arranged so as to conduct solid phthalic acid to a hopper 55 on the phthalic acid still 56. The hopper 55 may be provided with a suitable star valve 51 or other suitable means for controlling introduction of phthalic acid into the still 56.

'I'he phthalic acid still may be of any convenient construction and is shown as a. simple distillation vessel provided with a heating coil 56a. It preferably should be suitably insulated to avoid heat loss and promote uniformity of operation. The still has a vapor otake 58 with a branch 59 and a branch 60. These branches are equipped with valves 6I and 62. Branch 59 leads to the phthalic acid slurry tank 33 through a suitable distributor 63. The line 59 also may be provided with a check valve 64 for preventing back flow of liquid from the slurry tank to the phthalic acid still. Branch 60 leads to the base of a rectication column 65.

Column 65 may be of any suitable construction, such as a plate column or a bell and tray column, and has at the bottom thereof a valved liquid return line 66 and at the top a suitable heat exchanger 61 which may be provided with a liquid inlet 68 and outlet 69. This heat exchanger may be in the nature of a waste heat boiler, water being introduced at 6B and steam being Withdrawn at 69. By controlling the steam pressure, the temperature at the top of the column may be regulated. A vacuum connection 10 is provided at the top of the column. From the top plate of the column a liquid withdrawal line 1I having a valve 12 is provided for the withdrawal of phthalic anhydride. This withdrawal line may lead to apparatus for placing the phthalic anhydride in suitable physical condition for transportation or use or may lead to further purification apparatus `as desired. As shown in the drawing, it conducts liquid phthalic anhydride to a ilaker 13 having an inlet and outlet 14 and 15 for cooling water and a doctor 16 for scraping solid phthalic anhydride from the drum and conducting it to a storage bin 11.

It will be understood that parts of the equipment, which it is desired to maintain at elevated temperature, may be provided with suitable insulation to avoid excessive heat losses.

lene, the process may be carried out in the above apparatus as follows:

A body of naphthalene is maintained in molten condition in vaporizer I by means of heat applied by steam jacket 2. Additional naphthalene is introduced through inlet 4 to compensate for its removal by evaporation. Air is supplied through pump 21, pipe 26, heat exchanger 29, and pipe 3 to vaporizer I at a pressure around 3 atmospheres absolute. In the vaporizer I it bubbles up through the body of molten naphthalene and absorbs vapors therefrom. The resultant naphthaleneair mixture passes out from vaporizer I through conduit 5. Additional hot air may be introduced through by-pass 1 so as to reduce the naphthalene to air ratio to between 1 t 330 and l 400. The resultant mixture, which may be at a temperature around C., passes through conduit 5 to heatl exchanger 6 where it is heated by indirect heat exchange with hot reaction gases to a temperature around 330 to 400 C.

It may be noted here that in beginning operation of the above apparatus, air may be preheated in heater 29 to a high temperature say around 300 C., until a supply of hot reaction gases is available as the source of heat.

The hot reaction mixture passes from heat exchanger 9 through conduit I0 into primary converter Il, which may contain a vanadium oxide catalyst supported on a tray I2, for example a..

catalyst composed of about 10 parts of vanadium oxide disposed on parts of crushed silica brick.

In the catalytic converter the naphthalene is oxidized to phthalic anhydride with resultant rise of the temperature of the reaction mixture to between 525 and 550 C. The catalytic converter is operated adiabaticaliy, i. e. without any substantial heat flow thereto or therefrom except that introduced or Withdrawn by the gas stream and the temperature regulation is secured by controlling the ratio of naphthalene to entering gases.

The hot reaction gases are withdrawn from the primary converter and passed through conduit I3 to section I4a of heat exchanger 9 where they give up a portion of their heat to ingoing naphthalene airmixture. From this section of the heat exchanger they pass through a second section I4b in which any suitable cooling means is provided and by which their temperature is regulated to between about 400 and about 450 C. At this temperature they pass into secondary converter I8 which may contain a bed of oxidation catalyst similar to that in 'the preliminary converter and in this converter any naphthoquinone produced by the preliminary oxidation is converted to phthalic anhydride.

The reaction gases are withdrawn from secondary converter I8 at a. temperature around 430 to 480 C. and are passed through conduit I9 into heat exchanger 6 where they are cooled by indirect heat exchange With ingoing naphthalene air vapor mixture to a temperature around 200 C. At this temperature the reaction gases, still containing in vapor phase the phthalic anhydride product of the oxidation. pass through conduit 20 and distributor 2| into cooler 22 at a point well below the surface of a body of phthalic acid slurry in water containing around 15% to 30% of solid phthalic acid as crystalline slurry and maintained at a temperature between about 50 C. and about 60 C. The gases bubble up through the body of liquid and are cooled thereby to about the temperature of the cooling liquid. Phthalic anhydride reacts with the water to form phthalic acid and is retained in the veis cooling liquid in crystalline form. Any maleic anhydride or maleic acid .present-in the gases is simultaneously absorbed or dissolved and retained in the solution.

The gases freed from phthalic anhydride and maleic anhydride or maleic acid pass through outlet conduit 23 while yet at a pressure above about two atmospheres absolute, and are recirculated by means of pump 25 to the naphthalene vaporizer l. After the recirculation of gases is under way a portion thereof is bled off at 24 and a corresponding portion of air is introduced through inlet 28 by means of air compressor 21.

The pressure of the gases in the system is preferably maintained so that the gases pass through the catalytic converter at around 3 atmospheres absolute pressure, and preferably presy sure losses in the system are minimized so that a. reduction of pressure of only a fewtenths of one atmosphere takes place through the entire system. The ratio of additional air to recirculated gas maybe varied within wide limits. It is preferred, however, to adjust the bleed off of tail gas at 24 and the introduction of additional air at 28 so as to maintain an oxygen content of between and 10% by volume in the gases leaving converted I8 and the additional air supplied may be only about 1/th to 1lb-th of the total volume of gases passing through the converter. v

It will, of course, be noted that although the reaction mixture passing through the primary converter has been referred to as a mixture of naphthalene and air, once the apparatus is in proper operation, the mixture will comprise a small portion of added air and a relatively large portion of tail gases which in addition to oxygen and nitrogen will contain gaseous products of the reaction. A

Phthalic acid slurry is withdrawn through outlet pipe. 30 and pipe 3| to the tank 33 and passes thence through pipe 34 to flash ltank 31 maintained under vacuum by means of the jet evacuator 4'2. Resultant evaporation of water from the solution may cool the slurry to around 20 to 40 C. and this cooled slurry is returned via pipe 38 to the body of liquid in cooler 22 where it mingles with the. slurry containedtherein. The amount of liquid passing through this circuit may be controlled by the pump 39 so that a substantially constant temperature of about 50 C. lto 60 C. is maintained in the body of liquid in cooler 22. -A small portion of the liquid withdrawn through outlet 30, say 1% to 5% thereof,

yis Withdrawn through outlet pipe 41 to a drum iilter 48 where liquid is separated from solid by suction, liquid passing through pipe 50 to air separator 5|. The suction is maintained on the lter by an exhauster, not shown, connected to line 52. The main portion of thisliquid, free from solid phthalic acid, is returned via pipe 53v to the tank 33. A minor portion thereof may be bled olf through bleed line 54 for recovery of maleic acid from solution or for other suitable disposal, the amount of bleed being controlled so that maleic acid will not be present in the solution or slurry in cooler 22 in suflicient quantity to exist in solid phase under the conditions of illtration. Solid phthalic acid collects on the drum filter and is scraped therefrom by doctor 49 and conducted to hopper or bin 55 from whence it may be passed through valve 51 into a phthalic acid still 56.

'Ihe still illustrated is designed for intermittent operation and the hopper 55 provides storage for phthalic acid while a preceding charge is being treated in the. still. When sufficient phthalic acid has been introduced into still 56 to constitute a charge therefor,'the introduction of additional acid through valve 51 is discontinued and steam is passed through coil 56a to dehydrate the phthalic acid. During vthis opera- Ition the valves on lines 60 and 66 are kept closed and valve 6| is opened so rthat vapors from the still pass through conduits 58 and 59 to distributor 63 Withinthe slurry tank 33.` I-Iere the vapors bubble up through the slurry with the result that they are ywashed free from any phthalic acid which they may contain. When the charge of phthalic acid is completely dehydrated, which may be indicated by rise of temperature in the still appreciably above 190 C. when the still is operated at approximately atmospheric pressure, valve 6I is closed and the valves on lines 60 and 66 are opened so that Vaporsmay pass from the still up through column 65 and reflux liquid may return through 66 to the still. 'I'he temperature in the still is then further raised and vacuum is applied through connection 10 to reduce the pressure to around 1.7 pound absolute in the cblumn. Phthalic anhydride vfapors pass up through column 65 and are rectified by countercurrent contact with phthalic anhydride condensate formed in the condenser 61. The tem- L perature at the top tray of the column is controlled by suitable adjustment of the cooling fluid in condenser 61 so that a substantially pure phthalic anhydride product may be withdrawn in liquid phase through outlet 1|. Thus a temperature of about 200 C. may be maintained at the top of the column and the withdrawal of phthalic anhydride may be regulated so as to provide ample reiiux liquid to flo-W down through the column.

' a catalytic converter containing an oxidation catalyst, and regulating the ratio of naphthalene to oxidizing gas so as to maintain a-temperature between 450 and 600 C. in the catalytic converter.

`2. In the preparation of phthalic anhydride by the catalytic oxidation of naphthalene in vapor phase, the improvement which comprises passing a mixture of naphthalene vapor and oxidizing gas in a molar ratio between 1:200 and 12500 through a catalytic converter containing an oxidation catalyst and operated substantially adiabatically, regulating the ratio of naphthalene to oxidizing gas so as to maintain a temperature between 450 and 600 C. in the catalytic converter, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, ,bleeding oif a portion of the cooled gases, adding an oxygen-containing gas to the remainder, the oxygen content of said gas being substantially greater than that of said remainder, and using the resultant mixture as the oxidizing gas for phthalic anhydride.

The liquid lphthalic anhydride withdrawn may be passed to a flaker 13 and.'

3. In the preparation of phthalic anhydride by the catalytic oxidation of naphthalene in vapor phase, the improvement which comprises passing a mixture of naphthalene vapor and oxidizing gas in a molar ratio between 1:200 and 1:500 through a catalytic converter containing an oxidation catalyst and operated substantially adiabatically, regulating the ratio of naphthalene to oxidizing gas so as to maintain a temperature between 450 and 600 C. in the catalytic converter, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, bleeding o a portion of the cooled gases, adding air to the remainder, using the resultant mixture as the oxidizing gas for converting further quantities of naphthalene to phthalic anhydride, and maintaining the gases throughout the system at a pressure above 2 atmospheres absolute.

4. The method of preparing phthalic anhydride by the catalytic oxidation of naphthalene in vapor form, which comprises preparing a naphthaleneoxidizing gas mixture containing the naphthalene and oxidizing gas in a molar ratio between 1:200 and 1:500, passing the mixture heated to a tem-` perature between 300 and 350 C. into a catalytic converter containing an oxidation catalyst, operating said converter substantially adiabatically, regulating the ratio of naphthalene to oxidizing gas so as to maintain the temperature of the catalyst bed between about 450 and 600 C. whereby the naphthalene is oxidized to phthalic anhydride, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, bleeding off a portion of the residual gases, adding air to the remainder, and employing it as the oxidizing gas for oxidation of subsequent portions of naphthalene.

5. In the catalytic oxidation of naphthalene to phthalic anhydride, the improvement which comprises passing a mixture of naphthalene vapors and oxidizing gas through a catalytic converter at an elevated temperature to convert the naphthalene to phthalic anhydride, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydrides, bleeding off a portion of the residual gases, mixing the remainder with air, circulating the resultant mixture in admixture with naphthalene vapor into contact with the oxidation catalyst for effecting oxidation of further quantities of naphthalene, and maintaining a pressure above 2 atmospheres absolute on the gases throughout the system.

6. In the catalytic oxidation of naphthalene to phthalic anhydride, the improvement which comprises passing a mixture of naphthalene vapors and oxidizing gas through a catalytic converter at an elevated temperature to convert the naphthalene to phthalic anhydride, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, bleeding off a portion of the residual gases, mixing the remainder with air, circulating the resultant mixture in admixture with naphthalene vapor into contact with the oxidation catalyst for effecting oxidation of further quantities of naphthalene, and maintaining a pressure between 2 and 5 atmospheres absolute'on the gases throughout the system.

7. The method of effecting partial oxidation of naphthalene to phthalic anhydride, which cornprises passing a mixture of naphthalene vapor and oxidizing gas at a temperature between 300 and 350 C. in a molar ratio between 1:330 and 1:400 at a pressure between 2 and 5 atmospheres absolute into contact with an oxidation catalyst in a converter operated substantially adiabatically, regulating the ratio of naphthalene to air so as to maintain a temperature between 450 and 600 C. within the converter, cooling the hot reaction gases to condense phthalic anhydride therefrom, bleeding off a portion of the residual gases, adding additional air to the remainder, and using the resultant mixture as the oxidizing gas for oxidation of further quantities of naphthalene, said bleed off and air addition being regulated so as to maintain the oxygen content of the gases leav- 'ing the converter at 5% to 10% by volume.

8. The method of effecting partial oxidation of naphthalene to phthalic anhydride, which comprises passing a mixture of naphthalene vapor and oxidizing gas at a temperature between 300 and 350 C. in a molar ratio between 1:330 and 1:400 at a pressure between 2 and 5 atmospheres absolute into contact with an oxidation catalyst in a converter operated substantially adiabatically, regulating the ratio of naphthalene to air in the mixture so as to maintain a temperature between 450 and 600 C. within the converter, cooling the hot reaction gases to condense phthalic anhydride therefrom, bleeding off a portion of the residual gases, adding additional air to the remainder, passing the resulting mixture into contact with liquid naphthalene to vaporize the naphthalene and form a naphthalene vaporoxidizing gas mixture, regulating the naphthalene vapor to oxidizing gas ratio to between 1:330 and 1:400, heating the mixture to between 300 and 350 C., and passing the resultant hot mixture into the converter for the formation of phthalic anhydride.

9. The method of effecting partial oxidation of naphthalene to phthalic anhydride, which comprises passing a mixture of naphthalene vapor and oxidizing gas at a temperature between 300 and 350 C. in a molar ratio between 12330 and 1:400 at a pressure between 2 and 5 atmospheres absolute into contact with an oxidation catalyst in a converter operated substantially adiabatically, regulating the ratio of naphthalene to air so as to maintain a temperature between 450 and 600 C. within the converter, bringing the reaction gases while yet hot into intimate contact with an aqueous cooling liquid to remove phthalic anhydride, bleeding off a portion of the residual gases, adding additional air to the remainder, using the resultant mixture as the oxidizing gas for oxidation of further quantities of naphthalene, and maintaining the gases under a pressure between 2 and 5 atmospheres absolute throughout the system.

10. In the preparation of phthalic anhydride by the catalytic oxidation of naphthalene in vapor phase, the improvement which comprises passing a mixture of naphthalene vapor and oxidizing gas in a molar ratio between 1:200 and 1:500 through a catalytic converter containing an oxidation catalyst and operated substantially adiabatically, and regulating the ratio of naphthalene to oxidizing gas so as to maintain a temperature between 450 and 600 C. in the catalytic converter.

11. In the preparation of solid phthalic anhydride by the catalytic oxidation of naphthalene 'in vapor phase,'the improvement which comprises passing a mixture of naphthalene vapor and oxidizing gas in a molar ratio between 1:200 and 1:500 through a catalytic converter containing an oxidation catalyst and operated substantially adiabatically, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, bleeding off a portion of the cooled gases, adding an oxygencontaining gas to the remainder, and using the resultant mixture as the oxidizing gas for converting further quantities of naphthalene to phthalic anhydride, and regulating the addition of oxygen-containing gas so as to maintain the oxygen-content of gases leaving the converter at least about 5% by volume.

12. In the preparation of solid phthalic anhydride by the catalytic oxidation of naphthalene in vapor phase, the improvement which comprises passing a mixture of naphthalene vapor and oxidizing gas in a molar ratio between 1:200

and 1:500 through a catalytic converter containing an oxidation catalyst without substantial heat loss therein, withdrawing the hot reaction gases from the converter and cooling them to separate phthalic anhydride, bleeding oi a portion of the cooled gases, adding an oxygen-containing gas to th'e remainder, and using the resultant mixture as the oxidizing gas for converting further quantities of naphthalene to phthalic anhydride, and regulating the addition of oxygen-containing gas so as to maintain the oxygen-content of gases leaving the converter at 5% to 10% by volume.

DONALD A. ROGERS. FRANK PORTER. 

